Supermarkets' inconsistent use of "unit prices" on food and drink products is confusing cash-strapped shoppers and preventing them from working out which are cheapest, a consumer group has warned.

Research from Which? found that two-thirds of consumers have tried to compare the costs of the same type of products using unit prices, but only about half thought it was easy to work out which was better value for money.

Retailers are required by law to display both a selling price and a unit price on the shelf labels of food and drink products. The units that can be used are specified in legislation and include a kilogram, 100g, 10g, litre, 100ml or individual item.

Researchers from Which? visited branches of the main supermarkets in Glasgow, Leeds, London, Londonderry and Pembrokeshire to see how their unit pricing works in practice.

They found they were often not being displayed on multi-buys or promotions, and different units were used for varieties of the same product, making them extremely difficult to compare.

Some of the most confusing examples of unit pricing were found on fruit and vegetables. They included pre-packed red onions priced at £1.35 for three, or 45p an item next to loose onions priced at 95p a kilogram; a seven-pack of mini bananas with a selling price of 99p and a unit price of 14.1p each next to loose bananas priced at 68p a kilogram; and red peppers in 500g packs on sale for £2.09 and at £4.18 a kilogram next to packs of three mixed peppers priced at £1.65 or 56p each.

Which? said it was planning a major campaign on the issue, and planned to name stores and show specific examples.

Executive director Richard Lloyd said: "Unit prices should be a useful tool for people to compare food prices and choose the best value product, which is crucial when we're all looking to save money. But we've found unclear and inconsistent use of unit prices across all the major supermarkets."

He said Which? wanted retailers and the government to make urgent improvements so that consumers could compare like-for-like prices in the supermarket

The survey did not name out any individual retailers, but on behalf of the sector Andrew Opie, food director of the British Retail Consortium, said: "This week's food inflation figures show customers have no difficulty finding best value.

"The law gives shops the option of selling by weight or by unit for products, such as pre-packed fruit and vegetables. With some things - eggs, apples - how many they're getting matters more to customers than the overall weight."